The quality and performance of a digital fiber optic transmitter is determined by the distance over which the transmitted digital signal can propagate without severe distortions. The bit error rate (BER) of the signal is measured at a receiver after propagation through dispersive fiber and the optical power required to obtain a certain BER, typically 10−12, sometimes called the sensitivity, is determined. The difference in sensitivity at the output of the transmitter with the sensitivity after propagation is sometimes called dispersion penalty. This is typically characterized by the distance over which a dispersion penalty reaches a level of ˜1 dB. A standard 10 Gb/s optical digital transmitter, such as an externally modulated source, can typically transmit up to a distance of ˜50 km in standard single mode fiber at 1550 nm before the dispersion penalty reaches the level of ˜1 dB, which is sometimes called the dispersion limit. The dispersion limit is determined by the fundamental assumption that the digital signal is transform limited, i.e., the signal has no time varying phase across its bits and has a bit period of 100 ps, or 1/(bit rate), for the standard 10 Gb/s transmission. Another measure of the quality of a transmitter is the absolute sensitivity after fiber propagation.
Three types of optical transmitters are presently in use in prior art fiber optic systems: (i) directly modulated lasers (DML), (ii) Electroabsorption Modulated Lasers (EML), and (iii) Externally Modulated Mach Zhender (MZ) modulators. For transmission in standard single mode fiber at 10 Gb/s, and 1550 nm, it has generally been assumed that MZ modulators and EML can have the longest reach, typically reaching 80 km. Using a special coding scheme, sometimes referred to as phase shaped duobinary, MZ transmitters can reach 200 km. On the other hand, directly modulated lasers (DML) reach <5 km because their inherent time dependent chirp causes severe distortion of the signal after this distance.
By way of example, various systems for long-reach lightwave data transmission (>80 km at 10 Gb/s) through optical fibers which increase the reach of DML to >80 km at 10 Gb/s in single mode fiber are disclosed in (i) U.S. patent application Ser. No. 11/272,100, filed Nov. 8, 2005 by Daniel Mahgerefteh et al. for POWER SOURCE FOR A DISPERSION COMPENSATION FIBER OPTIC SYSTEM, (ii) U.S. patent application Ser. No. 11/441,944, filed May 26, 2006 by Daniel Mahgerefteh et al. for FLAT DISPERSION FREQUENCY DISCRIMINATOR (FDFD) and (iii) U.S. patent application Ser. No. 10/308,522, filed Dec. 3, 2002 by Daniel Mahgerefteh et al. for HIGH-SPEED TRANSMISSION SYSTEM COMPRISING A COUPLED MULTI-CAVITY OPTICAL DISCRIMINATOR, which patent applications are hereby incorporated herein by reference. The transmitter associated with these novel systems is sometimes referred to as a Chirp Managed Laser (CML™) by Azna LLC of Wilmington, Mass. In these new systems, a Frequency Modulated (FM) source is followed by an Optical Spectrum Reshaper (OSR) which uses the frequency modulation to increase the amplitude modulated signal and partially compensate for dispersion in the transmission fiber. In one embodiment, the frequency modulated source may comprise a Directly Modulated Laser (DML). The Optical Spectrum Reshaper (OSR), sometimes referred to as a frequency discriminator, can be formed by an appropriate optical element that has a wavelength-dependent transmission function, e.g., a filter. The OSR can be adapted to convert frequency modulation to amplitude modulation. See FIG. 1.
In the pending patent applications identified above, we have shown that the frequency modulation of the signal should be adjusted so that it is ˜½ times the bit rate of operation, e.g., approximately 5 GHz for a 10 Gb/s digital signal, as an example for extending the reach of the chirp managed laser transmitter in dispersive fiber. This prescription ensures that the relative phase of 1 bits separated by an odd number of 0 bits is π out of phase with each other, such that the bits interfere destructively in the middle of the 0 bit after they propagate in dispersive fiber.